Adiabatic triplet state tautomerization of p-hydroxyacetophenone in aqueous solution

The primary photophysical processes of p-hydroxyacetophenone (HA) and the ensuing proton transfer reactions in aqueous solution were investigated by picosecond pump-probe spectroscopy and nanosecond laser flash photolysis. Previous studies have led to mutually inconsistent conclusions. The combined data allow us to rationalize the excited-state proton transfer processes of HA in terms of a comprehensive, well-established reaction scheme. Following fast and quantitative ISC to the triplet state, (3)HA*, adiabatic proton transfer through solvent water simultaneously forms both the anion, (3)A(-)*, and the quinoid triplet enol tautomer, (3)Q*. The latter subsequently equilibrates with its anion (3)A(-)*. Ionization and tautomerization are likely to compete with the desired release reactions of p-hydroxyphenacyl photoremovable protecting groups.     

Identification of a TGBA liquid crystal phase via its defects

We have shown that cholesteryl nonanoate, a thermotropic compound which is well known to exhibit pretransitional effects at the smectic A (SmA) cholesteric (N*) transition (W.L. McMillan, Phys. Rev. A 4, 1238 (1971); 6, 936 (1972)), has in fact a TGBA phase in between. Our arguments rely on the observation of new TGBA defects, either in Robinson spherulites cooled from the N* phase or in free-standing films. The same new defects can be obtained in a well-documented TGBA phase of a tolane compound. We analyze qualitatively the TGBA defects in both geometries, in particular their relation to the disclination radius of the N* Robinson spherulites. Received 12 February 2001     

Universal properties of lamellar systems under weak shear

We have observed that different lamellar phases (thermotropic, lyotropic of surfactants or blockcopolymers) share the same rheological properties = A(T)σ^m, m = 1.67±0.1, independently of their chemical nature, in a range of shear rates , shear stresses σ, and temperatures T, where the flow is stationary. It is argued that the key phenomenon is the appearance of screw dislocations whose glide under an applied shear counterbalances plastically the applied vorticity, and stabilizes the layers, whose slip past each other is thus made more feasible. The theory, which makes use in other respects of the high-temperature creep model in solids, reproduces correctly the power law behavior and the values of A(T). Received 18 April 2001 and Received in final form 25 October 2001     

3H-Phosphaallenes Revisited: Facile Synthesis by Hydroalumination of Alkynylphosphines and β-Elimination,Stability and Trapping of Transient Species by Coordination to Transition Metal Atoms

A facile method for the efficient synthesis of 3H-phosphaallenes, R−P=C=C(H)−R′, is presented, which comprises treatment of dialkynylphosphines with dialkylaluminium hydrides (hydroalumination) and elimination of aluminium alkynides from intermediate alkenyl-alkynylphosphines. The stability of the phosphaallenes depends on steric shielding by the substituents at phosphorus (aryl or CH(SiMe₃)₂ groups). Only supermesityl compounds are persistent at room temperature in solution. This simple method starting with easily accessible dialkynylphosphines and commercially available aluminium hydrides (HAlEt₂, HAliBu₂) allows the generation of transient species, which were trapped by coordination to transition metals. The η¹-coordination via a P−W bond was observed for tungsten, while the side-on coordination via the P=C bond resulted with platinum. Decomposition of the mesityl derivative yielded an unprecedented product, which may be formed by 1,3-H shift to the P atom, hydrophosphination of the P=C bond of a second phosphaallene and formation of a P−P bond.     

Exact groundstates for antiferromagnetic spin-one chains with nearest and next-nearest neighbour interactions

We have found the exact ground state for a large class of antiferromagnetic spin-one chains with nearest and next-nearest neighbour interactions. The ground state is characterized as a matrix product of local site states and has the properties characteristic of the Haldane scenario.Work performed within the research program of the Sonderforschungsbereich 341, Köln-Aachen-Jülich     

The two-proton shell gap in Sn isotopes

We present an analysis of two-proton shell gaps in Sn isotopes. As theoretical tool we use self-consistent mean-field models, namely the relativistic mean-field model and the Skyrme-Hartree-Fock approach, both with two different pairing forces, a delta interaction (DI) model and a density-dependent delta interaction (DDDI). We investigate the influence of nuclear deformation as well as collective correlations and find that both effects contribute significantly. Moreover, we find a further significant dependence on the pairing force used. The inclusion of deformation plus correlation effects and the use of DDDI pairing provides agreement with the data.     

Shape coexistence near the double-midshell nucleus 111Rh

The decay of ¹¹¹Ru obtained from fast on-line chemical and mass separation has been investigated by β-γ-t and γ-γ coincidence techniques. Earlier spin and parity assignments of ¹¹¹Rh levels based on extrapolations of level systematics are confirmed. In particular, the K=1/2 intruder band is supported by the hindrance of E2 transitions between deformed and spherical states and enhancement of intraband E2 transitions. The excitation energies of intruder band members in Rh isotopes show a minimum at ¹⁰⁹Rh₆₄, with two neutrons less than ¹¹¹Rh at the N=66 midshell. This trend, which differs from the one in the higher-Z neighbouring elements Ag and Cd with minima at N=66, follows the evolution of deformation observed in the lower-Z elements Ru and Mo. Received: 25 June 1997 / Revised version: 10 September 1997     

Therapeutic monitoring of metipamide during antihypertensive therapy

The aim of our study was to monitor metipamide during a two-month period of treatment and to determine whether the whole-blood levels estimated by high-performance liquid chromatography provide a relevant indicator of possible accumulation of the drug. We also analysed antihypertensive activity and biochemical changes in the blood of twenty hypertonic patients. The results of our clinical trial showed that metipamide is an effective first-line antihypertensive agent, in that it combines satisfactory reduction of blood pressure with a low frequency of side-effects and a simple once-daily dosage regime.     

The space-time structure of quantum systems in external fields

An axiomatic foundation of a quantum theory for microsystems in the presence of external fields is developed. The space-time structure is introduced by considering the invariance of the theory under a kinematic invariance group. The formalism is illustrated by the example of charged particles in electromagnetic potentials. In the example, gauge invariance is discussed.     

Quantum phase transition in spin-3/2 systems on the hexagonal lattice — optimum ground state approach

Optimum ground states are constructed in two dimensions by using so called vertex state models. These models are graphical generalizations of the well-known matrix product ground states for spin chains. On the hexagonal lattice we obtain a one-parametric set of ground states for a five-dimensional manifold of S = 3/2 Hamiltonians. Correlation functions within these ground states are calculated using Monte-Carlo simulations. In contrast to the one-dimensional situation, these states exhibit a parameter-induced second order phase transition. In the disordered phase, two-spin correlations decay exponentially, but in the Néel ordered phase alternating long-range correlations are dominant. We also show that ground state properties can be obtained from the exact solution of a corresponding free-fermion model for most values of the parameter.